1. Field of the Invention
The invention pertains to switching optical signals and more particularly to switches for bypassing a defective data station in a serial network of data stations wherein data is transmitted via optical signals.
2. Description of the Prior Art
High information transmission capacity, immunity to electromagnetic interference, and freedom from ground loop problems ideally suit optical transmission systems for linking distributed computers, computer controlled industrial components, and other data transmission systems. These optical transmission systems utilize optical fibers to serially link a multiplicity of optical repeater stations. A power failure at one of the serial link stations, however, may interrupt the data chain and cause the entire system to fail. To prevent such a catastrophe, a fail safe optical switch is employed at each repeater which operates to bypass that station when a fault occurs, as for example, a power loss. These fail safe switches must possess low insertion loss properties, and provide high isolation between the input and output optical fibers during the "Power On" mode. Many such networks have hundreds, if not thousands of data stations each requiring a bypass switch, making the cost of the by-pass a major factor.
Conventionally, the optical switches utilized have been mechanical in nature. Mechanical switches, though relatively inexpensive, inherently include moving parts and generally require high driving power. These moving parts are subject to wear, abrasion, fatigue and other mechanical stresses and as a consequence are themselves prone to failure.
Optical switches, utilizing a liquid crystal material, as the optical signal direction control mechanism have been proposed. At present, however, these proposed liquid crystal switches are both expensive and difficult to mass produce. As proposed, these devices employ a series of triangular prisms, having optically flat surfaces. These prisms are difficult to manufacture and represent the bulk of the manufacturing costs of the switch. Further manufacturing difficulty arises due to the requirement that the optically flat bases of the prism be parallel and laterally aligned to insure that the path of the light beams passing therethrough maintain a prescribed path.
Other types of optical switches in the prior art utilize a Faraday rotator comprising YIG crystal to effect polarization rotation of the optical signal and a polarization separator to accomplish the desired switching. These switches exhibit excessive inertia due to the wiring in an electromagnet required to establish the necessary magnetic field about the YIG to produce the polarization rotation. Additionally, large amounts of electrical current must pass through the coils to establish the required magnetic field. The current may be reduced somewhat with additional turns of wire, but this adds to the inertia of the switch. Further, the YIG crystal is constructed as a slab optical waveguide and presents an interface problem with the optical fibers of the data system.
Another bypass switch of the prior art utilizes PLZT wafers to which an electrical voltage is applied to effectuate a polarization rotation. This switch, as do the other polarization sensor devices, requires polarization beamsplitters to direct the polarized light, and collimating and focussing lenses for interfacing the PLZT wafers with the optical fibers. In addition to requiring the high voltage to provide the necessary polarization rotation, the PLZT wafers are difficult and expensive to manufacture. Further, the necessary electrode through which the wafer voltage is applied must be positioned on the wafer clear of the light path, adding to the cost and size of the manufactured switch.